1. Field of the Invention
The present invention relates to a touch-responsive apparatus in an electronic musical instrument provided with a keyboard, and more particularly to such a touch-responsive apparatus which controls musical tones by detecting an after-touch such as depth or pressure of a key depression following or upon its complete depression.
2. Description of the Prior Art
There has been practiced so-called touch-responsive control for controlling tone volume, tone color and/or tone pitch in accordance with a key touch such as speed or depth of a key depression in order to improve the performance function of an electronic musical instrument. As one form of such touch-responsive control, there has been proposed a method such that the state of a key touch or pressure after it has been completely depressed once (such a key touch will hereinafter be called "after-touch") is detected, for example, by an after-touch sensor which is provided under the keyboard, to thereby control the tone volume, the tone color and/or the tone pitch of a musical tone (such a control will hereinafter be called "after-touch control").
As the above-mentioned after-touch sensor, there has been used, in general, a piezoelectric device or an electro-conductive rubber. An after-touch sensor which employs an electro-conductive rubber is disclosed in, for example, U.S. Pat. No. 3,784,935. An output signal of this after-touch sensor is utilized in elevating a sustain level, or in intensifying vibrato, or in varying a tone color by varying the filter cut-off frequency or the frequency modulation index which determines the spreading of harmonics.
Now, the output of an after-touch sensor, in general, is relatively gentle in its rise. Therefore, in case the volume of a musical tone is controlled by relying on the output of the after-touch sensor, there is brought about a delay in starting the sounding of the musical tone, so that there has been the problem that the sounding of musical tones tends to become unnatural.